1) Yes yes yes. They're waiting for you.
2) Probably best to use a seperate vessel. Blend with your cold water, get your 75 degree wort, then pour that into your carboy with the yeast cake. You'll get extra aeration from the second pour, consider that a bonus.
Or, another way to tackle this... pour in about 1 qt of your cold top off water into the carboy with the yeast cake, and pour the rest into another carboy with your wort as normal. Swirl the yeast cake up really well with the 1qt of cold water you added, then pour the yeast into the new (clean) fermenter with all the wort in it.
I like to pour the wort onto the yeast cake, though, because it gives me pretty decent aeration for the whole batch.
+1 to jim, I would chill this batch a little further in the ice bath, before you start with the top-up water. With an active yeast cake, ideally you'd want to be around 60*F and come back up.